Cucurbit Downy Mildew: A Traveling Menace to Squash, Cucumbers, and Melons

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Some plant diseases that affect area vegetable gardens survive in our soil, on plant residues, or on weedy hosts. These tend to infect our plants early in the season since they remain present in the garden year-round and reproduce and spread once conditions are favorable and a susceptible plant victim is present. Others do not survive the winter outdoors in our climate. They must travel in each year with weather systems from warmer locales or they may survive the winter in a heated greenhouse. One of these traveling threats that arrives in North Carolina virtually every year is Cucurbit Downy Mildew.

Cucurbit Downy Mildew is caused by the fungal-like Pseudoperonospora cubensis. It affects members of the Cucurbitaceae family, or squash family, including cucumbers, squash and pumpkins, and melons. There are two types, or ‘clades’ that may appear in a given year. Clade 1 more frequently infects watermelon, squash and pumpkins, and clade 2 tends to infect cucumbers and cantaloupe primarily. Each year we may get one or both of these in North Carolina. They often travel in with weather events and frequently are first identified in June or July.

Downy mildew causes angular, yellow spots to occur on leaf surfaces. These spots sometimes have fuzzy, purplish downy growth on the underside of the leaves, which are the spore masses, or sporangia- the source of spread and new infection. Downy mildew reproduces and spreads best when cool temperatures prevail- 60s are ideal- and there are 6-12 consecutive hours of leaf wetness caused by rain, irrigation, or dew each day.

There are several foliar diseases which also cause leaf spots and may be confused with downy mildew. These include angular leaf spot, anthracnose, and Alternaria leaf blight. Powdery mildew is frequently seen on members of the cucurbit family. Instead of discrete yellow or brown spots, a powdery substance forms on the tops of leaves. Leaf edges may also turn brown. One difference between downy mildew and the other diseases is that downy mildew will spread extremely rapidly over an entire planting and may kill the plant or cause it to stop producing very quickly. However, left unmanaged, all of these foliar diseases can kill a plant or greatly reduce yields.

There is reporting network which farmers and gardeners can use to track the movement of Cucurbit Downy mildew, and receive notifications when it approaches the area. Track progress and sign up for the alerts at: cdm.ipmpipe.org. The closest confirmed case this year was in Davie County on July 24, 2020.

Gardeners are advised to take precautionary steps every year to reduce foliar diseases of cucurbits. Look for varieties with tolerance to downy mildew- more of these are becoming available to home gardeners. Citadel and Peacemaker are pickling varieties with tolerance to clade 2 downy mildew. DMR 401 is a new slicing variety exhibiting good resistance to downy mildew. There are many squash, pumpkin, and some cucumbers with tolerance or resistance to powdery mildew. Tolerance means that they may get the disease and show symptoms, but yield is not as affected by its presence. Resistance to a disease means that a plant may suffer little infection and exhibit few symptoms, even if the disease is present and conditions are favorable.

Additional steps to reduce disease include mulching, proper watering, and applying preventive fungicides before the disease is present or when it is first visible. Mulch gardens with at least 3” straw, landscape fabric, plastic, cardboard, or a combination to reduce weeds and soil splash onto leaves. Avoid overhead watering or irrigation. Aim to water in the early morning, when dew is present but will dry later in the day.

Plan to apply protectant fungicides with the active ingredients copper and chlorothalonil. These are the only two that can prevent downy mildew and the other leaf spot diseases. Organic gardeners can use fixed copper products such as copper soap. Sulfur is an effective fungicide for preventing powdery mildew, but will not control the other diseases. Sulfur and copper can cause leaf damage or phytotoxicity in plants when applied during certain weather conditions. Avoid applying sulfur when temperatures are above 80°F – this usually restricts timing to late evenings. Copper should generally be applied when leaves will dry quickly. Be sure to thoroughly read and follow all label instructions regarding rates, timing, temperatures, and personal protective equipment (PPE) before applying any pesticide.

Finally, be sure to practice good crop rotation and sanitation in your garden to minimize disease carryover from year to year. Avoid planting members of the Cucurbitaceae family in the same place they were in recent years. Till in or remove all crop residues at the end of the season. Regular additions of organic matter such as compost may also be beneficial in reducing disease and promoting plant health.